Genetic Dissection of OXPHOS Assembly in Skeletal Muscles The skeletal musculature is by far the largest organ in animals, accounting for about half the body weight of humans and up to 75% of the body mass of insects such as Drosophila. In order to provide the energy required for contraction of muscles, skeletal muscles tend to be highly enriched with mitochondria. Accordingly, mitochondrial disorders frequently present with myopathy as a prominent clinical feature. While the factors responsible for increasing overall mitochondrial mass during myogenesis have been well-characterized, relatively little is known about the specific factors that assist with assembling the Oxidative Phosphorylation (OXPHOS) complexes in muscles. The overall goal of this project is to identify novel factors that regulate OXPHOS assembly in Drosophila flight muscles. When genes that regulate the assembly of OXPHOS complexes are genetically disrupted in Drosophila flight and leg muscles, they cause a readily discernible locomotory defect that can easily be scored and quantified. Using this phenotypic read-out we are genetically screening a focused set of mitochondrial proteins for novel regulators of OXPHOS assembly in flight muscles. We have shown that OXPHOS assembly commences soon after the flight muscles are made, and occurs via a mechanism that is very similar to what has been described in mammalian systems. Importantly, mutations in Drosophila orthologs of OXPHOS assembly factors described in humans, also impair OXPHOS assembly in Drosophila flight muscles, further showing that the mechanism of OXPHOS assembly is conserved between humans and Drosophila. We will screen for genes that cause locomotory defects when disrupted in muscles (Aim 1), and determine whether they regulate OXPHOS assembly using several well-established mitochondrial assays (Aim 2). Finally, we will perform additional tests to confirm our candidate regulators in skeletal muscles from mice (Aim 3). Altogether, this analysis is likely to uncover new regulators of OXPHOS assembly in muscles; which will lead to a better understanding of the pathology of primary mitochondrial myopathies, and possibly uncover novel therapeutic opportunities.